Processes are already known for the preparation of stable, aqueous polyurethane-polyurea dispersions (e.g. German Patent Nos. 1,184,946 and 1,178,586, German Auslegeschrift No. 1,237,306 and German Offenlegungschrift Nos. 1,495,745; 1,595,602; 1,770,068; 2,019,324 and U.S. Pat. Nos. 3,388,087, 3,480,592, 3,461,103, 3,479,310, 3,756,992, 3,686,108 and 3,905,929; see also D. Dieterich et al, Angew.Chem. 82,53 (1970) ). The described dispersions are based on the principle of building hydrophilic centers into a macromolecular chain of a polyurethane (polyurea) molecule. The hydrophilic centers or so-called "internal emulsifiers" in these known dispersions are segments which have ionic groups or ethylene oxide groups. These hydrophilic centers are either built into the prepolymer in the form of certain diols or used as modified amines for chain lengthening the prepolymers which always have at least two isocyanate end groups.
Various processes may be used for preparing the known dispersions and have been described, for example, in D. Dieterich and H. Reiff, Angew. makromol. Chemie 26,85 (1972). As a general rule, a solution of a polyurethane in an organic solvent is either converted into an aqueous dispersion or a liquid prepolymer stage is dispersed in water, either with or without solvent. For example, a liquid prepolymer ionomer containing isocyanate groups may be introduced into water with vigorous stirring. In that case, an emulsion of the prepolymer is initially formed which undergoes further reaction with water or a diamine or polyamine under conditions of chain-lengthening to form the high molecular weight polyurethane urea.
One method of dispersion which is particularly simple to carry out has been described in German Offenlegungsschrift No. 1,913,271 (or U.S. Pat. No. 3,756,992). According to this method, a solid or liquid polyurethane polyelectrolyte which is capable of undergoing addition with formaldehyde to form methylol groups is dispersed by mixing it with water and is converted into a methylol group containing polyurethane polyelectrolyte by the addition of formaldehyde or a formaldehyde derivative. The polyurethane polyelectrolyte is then condensed to form the high molecular weight polyurethane either in the dispersion or on a substrate.
To bring about dispersion, water is added to the stirrable melt until it forms the continuous phase. The system generally passes through the stage of a water-in-oil emulsion before this is achieved. Alternatively, a water-in-oil emulsion may be prepared at elevated temperature, this emulsion changing into an oil-in-water emulsion on cooling. Preparation of the dispersion is generally carried out at elevated temperatures, preferably at from about 50.degree. to 120.degree. C. This is necessary, firstly because the prepolymer stage which is to be dispersed is too highly viscous at room temperature so that expensive apparatus, such as screw extruders would be necessary for dispersion, and, secondly because the rate at which dispersion progresses generally increases with increasing temperature. Dispersion of solvent-free melts of isocyanate prepolymers is also generally carried out at elevated temperatures.
Although the method described above is economically very advantageous when compared with the preparation of dispersions from solutions and may be carried out by simple means, it does not satisfy all practical requirements. Non-ionic dispersions are difficult or even impossible to prepare by this method because the dispersibility of preliminary stages of polyurethanes which contain hydrophilic polyether groups instead of ionic centers decreases with increasing temperature. This means that the compounds must be rendered more hydrophilic if they are to be dispersed at the desired elevated temperatures than is necessary for the subsequent stability of the dispersion. This means that the water-resistance of the products is deleteriously affected. On the other hand, the preparation of polyurethanes modified by hydrophilic ether groups would be particularly desirable because such dispersions have high resistance to electrolytes and to frost.
The conventional ionomer dispersions are generally insufficiently stable to electrolytes and to frost so that storage and transport during cold weather are expensive because they require heated storage rooms.
It has now surprisingly been found that non-ionic dispersions which contain hydrophilic polyether groups built into the molecule as emulsifier and which are capable of being cross-linked may be prepared very simply and without the aid of solvents if prepolymers containing isocyanate end groups and having a molecular weight of below about 15,000 and containing from about 2 to 30%, by weight, of ethylene oxide units arranged within a polyether chain are reacted with ammonia or primary amines under particular conditions which will be defined in detail below and the major proportion of the water required for dispersion or solution is then added.
Although an indication is given in U.S. Pat. No. 3,756,992 (column 18, lines 25-38) that ionic prepolymers having isocyanate end groups may be converted by reaction with ammonia or primary amines into the corresponding ionic adducts which are dispersible in water and which contain urea groups capable of being cross-linked, it has hitherto always been considered necessary to introduce the amine into the reaction vessel first or to add it very rapidly in stoichiometric excess with vigorous mixing in order to prevent unwanted chain lengthening and hence unwanted increase in viscosity.
At first, it seemed impossible to apply this principle to polyurethanes which contained non-ionic hydrophilic centers because all attempts to do so failed. Thus, the attempt to add isocyanate prepolymers to ammonia or primary amines in the reaction vessel was as unsuccessful as the attempt to reduce the reactivity of ammonia by converting it into ammonium carbonate. The desired result was, however, achieved quite unexpectedly by the addition of aqueous ammonia or the addition of primary amines optionally dissolved in water to non-ionic modified isocyanate prepolymers. The success of this experiment was completely surprising since a large excess of isocyanate was present at least at the beginning of the addition of ammonia solution or of amine so that the unwanted chain lengthening reactions between the isocyanate prepolymer and the ammonia or primary amine which was quite uninhibited in its reactivity was to be expected.